Dimers of aromatic compounds are useful as starting materials for various industrial products. For example, alkali metal salts of 3,4,3', 4'-biphenyltetracarboxylic acids are very useful as starting materials for the production of heat-resistant polyimide resins.
A method is known to produce biphenyl compounds from aromatic halide compounds by dehalogeno-dimerizing them in the presence of a palladium catalyst, water and methanol (see Japanese Patent Examined Publication No. 14015/84). Various improvements of this method have been proposed, which comprise replacing methanol by polyhydric alcohols or formaldehyde [see Japanese Patent Publication (OPI) No. 26238/87 (the term "OPI" as used herein means an "unexamined published Japanese patent application")], formic acid or formates [see Synthesis Communications, 538 (1978), as well as Japanese Patent Publication (OPI) Nos. 137838/86 and 167642/86], and carbon monoxide [see Japanese Patent Publication (OPI) No. 293932/86]. However, even these modifications are insufficient to attain the desired biphenyl compounds in satisfactorily high yields and further improvements in this respect have been desired.
The present inventors conducted intensive studies in order to develop a process for producing biaryl compounds in high yield by dehalogeno-dimerization of aromatic halide compounds. As a result, the present inventors found that in the known method of dehalogeno-dimerizing aromatic halide compounds using palladium catalysts on supports, biaryl compounds could be produced in high yield when the palladium catalysts on supports had been immersed in hydrohalogenic acids such as hydrochloric acid prior to the dehalogenodimerization reaction.